Recently new catalysts have been developed which are far more active than conventional unsupported titanium halide organoaluminum systems catalyst in the polymerization of alpha-olefins. Briefly described, these new catalysts are comprised of a titanium halide catalyst component supported on magnesium dihalide and an alkylaluminum compound, which can be present as a complex with an electron donor compound. These catalyst components have been described in the patent literature, e.g. in U.S. Pat. Nos. 3,830,787, 3,953,414, 4,051,313, 4,115,319 and 4,149,990.
The productivities obtained with these new catalysts are extremely high resulting in polymers containing such small quantities of residual catalyst that the conventional deashing step can be dispensed with. The catalysts function well in the homopolymerization of propylene and in the copolymerization of a mixture of propylene and another alpha-olefin such as ethylene, especially when the polymerization reaction is carried out in a liquid diluent, e.g., liquid propylene monomer ("liquid pool" polymerization).
In large scale, continuous liquid pool copolymerization of ethylene and propylene there is a practical upper concentration limit of ethylene that can be incorporated into the copolymer before operational problems are encountered, e.g., in the filter separation of the product from unreacted monomer. These problems are due to less than acceptable isotactic indices (II) of the copolymer products. Since an increase in polymerized ethylene content is generally accompanied by a decrease in isotactic index (=% heptane insolubles) it has hitherto been the practice to limit the polymerized ethylene content to about 2-3 wt %. Although such copolymers of relatively low ethylene contents have found many uses and can be fabricated into a variety of articles of manufacture, their application in the fabrication of heat sealable films has been precluded because of the relatively high melting points of low ethylene-content copolymers.
It is, therefore, an object of this invention to provide an improved process for the liquid pool production of polymers of ethylene and propylene.
It is another object to improve the reactivity of ethylene in a liquid pool polymerization process for the production of polymers of ethylene and propylene.
It is a further object to provide a novel polymer of ethylene and propylene having an enhanced polymerized ethylene content as well as acceptable isotacticity.
It is still another object to provide a polymer of ethylene and propylene suitable for fabrication of heat sealable films.
These and other objects will become apparent from a reading of the specification and the appended claims.